The present invention relates generally to silicon wafer processing, and more particularly, to devices for holding silicon wafers as they are subjected to ion bombardment and to heat treatment.
Various techniques are known for processing silicon wafers to form devices, such as integrated circuits. One technique includes implanting oxygen ions into a silicon wafer to form buried layer devices known as silicon-on-insulator (SOI) devices. In these devices, a buried insulation layer is formed beneath a thin surface silicon film. These devices have a number of potential advantages over conventional silicon devices (e.g., higher speed performance, higher temperature performance and increased radiation hardness). The lesser volume of electrically active semiconductor material in SOI devices, as compared with bulk silicon devices, tends to reduce parasitic effects such as leakage capacitance, resistance, and radiation sensitivity.
In one known technique, known by the acronym SIMOX, a thin layer of a monocrystalline silicon substrate is separated from the bulk of the substrate by implanting oxygen ions into the substrate to form a buried dielectric layer. This technique of "separation by implanted oxygen" (SIMOX), provides a heterostructure in which a buried silicon dioxide layer serves as a highly effective insulator for surface layer electronic devices.
In the SIMOX process, oxygen ions are implanted into silicon, after which the material is annealed to form the buried silicon dioxide layer or BOX region. The annealing phase redistributes the oxygen ions such that the silicon/silicon dioxide boundaries become more abrupt, thus forming a sharp and well-defined BOX region, and heals damage in the surface silicon layer caused by the ion bombardment.
During the SIMOX process, the wafers are subjected to relatively severe conditions. For example, the wafers are typically heated to temperatures of about 500-600 degrees Celsius during the ion implantation process. Subsequent annealing temperatures are typically greater then 1000 degrees Celsius. In contrast, most conventional ion implantation techniques do not tolerate temperatures greater than 100 degrees Celsius. In addition, the implanted ion dose for SIMOX wafers is in the order of 1.times.10.sup.18 ions per square centimeter, which can be two or three orders of magnitude greater than some known techniques.
Conventional wafer holding devices are often incapable of withstanding the relatively high temperatures associated with SIMOX processing. Furthermore, wafer-holding structures having exposed metal are ill-suited for SIMOX processes because the ion beam will induce sputtering of the metal and, thus, result in wafer contamination. In addition, the structure may deform asymmetrically due to thermal expansion, which can damage the wafer surface and/or edge during high temperature annealing so as to compromise wafer integrity and render it unusable.
Another disadvantage associated with certain known wafer holders is electrical discharge of the wafers. If a wafer holder is formed from electrically insulative materials, the wafer will become charged as it is exposed to the ion beam. The charge build up disrupts the implantation process by stripping the ion beam of space charge neutralizing electrons. The charge built-up on the wafer can also result in a discharge to a nearby structure via an electrical arc, which can also contaminate the wafer or otherwise damage it.
It would, therefore, be desirable to provide a wafer holder that is electrically conductive and is able to withstand the relatively high temperatures and energy levels associated with SIMOX wafer processing while also minimizing the potential for sputter contamination.